Recently, in the cordless and portable trend of battery-operated appliances such as audio and video devices and personal computers, batteries of small size, light weight, and high energy density are demanded as their driving power sources. In particular, the lithium secondary battery is high in energy density, and is highly expected as the battery for next generation, and its latent market scale is great. Besides, from the viewpoint of thin design of appliances and effective use of space, there is a mounting demand for thin type of battery.
However, the enclosed type battery, for example, the lithium secondary battery using lithium metal or carbon material as negative electrode has been known to have a problem of generation and accumulation of gas in the battery due to decomposition of electrolyte or active material when falling into a state of short-circuit, overcharging, or reverse charging.
In the event of such short-circuit, overcharging, or reverse charging, to prevent explosion of battery, an explosion-proof device has been proposed, for example, as disclosed in Japanese Laid-open Patent 2-112151. That is, as shown in FIG. 12, a reed 202 is fitted to an explosion-proof valve 201 which is deformed as the internal pressure builds up, and when the internal pressure reaches a specified value, the reed 202 is separated from the explosion-proof valve 201 or torn apart, and the current is cut off. In FIG. 12, the explosion-proof valve 201 is installed in a housing 203 of the battery through a gasket 204.
In such constitution, however, when reaching the specified internal pressure, if the explosion-proof valve 201 is not securely set apart from the reed 202, or if a burr is formed by tearing, although the explosion-proof valve 201 has functioned, the conductive state between the explosion-proof valve 201 and reed 202 may be kept, and the state of short-circuit or the like may persist.
Or, when the explosion-proof device of such cylindrical battery is applied in an elliptical sealing plate of small width dimension as used in thin type battery, since the effective receiving surface of gas pressure is small and the width is narrow in the elliptical sealing plate, deformation of the metal explosion-proof valve by elevation of internal pressure is insufficient, and the explosion-proof valve may fail to work in spite of high internal pressure of battery. Besides, in the thin type battery, since the explosion-proof pressure of the battery case is lower than in the cylindrical battery, the operating pressure of the explosion-proof valve must be set lower than in the cylindrical battery. Accordingly, it is required to reduce the thickness of the elastic metal explosion-proof valve so as to increase deformation of explosion-proof valve due to elevation of internal pressure.
Moreover, in the state of low internal pressure of battery, if the thickness of the explosion-proof valve is reduced in order to deform the metal explosion-proof valve sufficiently, fluctuations occur in the operating pressure of the explosion-proof valve, and the explosion-proof valve may not function securely.
On the other hand, an explosion-proof device as shown in FIG. 11 has been also proposed. In FIG. 11, a part of an explosion-proof valve 301 is electrically connected with the upper part of a protrusion of a terminal plate 302 through a fusion part 303. As gas is accumulated in the battery, the explosion-proof valve 301 is deformed upward, and when the internal pressure reaches a specified value, the fusion part 303 is torn apart, and the terminal plate 302 and explosion-proof valve 301 are separated from each other, so that the explosion-proof valve 301 is electrically disconnected from the terminal plate 302.
In this structure, too, when the internal pressure of the battery climbs up, due to insufficient deformation of the metal explosion-proof valve and uneven fusion strength, especially if reaching up to the specified battery internal pressure, the central fusion part of the explosion-proof valve may not be torn apart, and the electric connection in the battery may not be cut off securely.
In the thin type battery, yet, since the width direction dimension is small and the strength is weak, when sealing by crimping, the parts contained inside may be deformed, and since gas is discharged, the thin wall part of electric connection part may often fall in trouble.